Amorphous In-Ga-Zn-O (a-IGZO) semiconductors have been used as an active channel material in high mobility, flexible, and transparent thin film transistors (TFTs), but it is highly influenced by the external environment. Many factors could affect TFT performance, such as moisture, oxygen, and hydrogen, etc. Among them, hydrogen incorporation into the oxide semiconductor is unavoidable due to the high electronegativity of oxygen. Hydrogen incorporated into a-IGZO acts as a shallow donor by ionizing and bonding with oxygen to form hydroxyl groups, so that it makes the oxide TFT conductive and causes it not to show an on/off property.[1] Hydrogen can be incorporated through various processes, such as diffusion from ambient atmosphere, incorporation during the deposition of encapsulation layer, and diffusion from adjacent layer during post-annealing, etc. In particular, it has been observed that a large amount of hydrogen atoms produced by SiH₄ plasma is incorporated into the a-IGZO layer, creating a shallow electron donor, when PE-CVD SiN_x layer is deposited for encapsulation. [2] Thus, an appropriate hydrogen barrier is essential to prevent the inflow of hydrogen to the a-IGZO layer during the subsequent process. Therefore, a method of preventing hydrogen incorporation by chemical combination should also be performed. However, there is no systematic study to prevent hydrogen incorporation. The analysis of hydrogen atomic behavior in thin films with different properties have not been clearly examined so far, thus further studies for this are necessary.

Techniques commonly used for the deposition of gas diffusion barrier deposition are sputtering, chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among them, ALD is considered as the most promising method to produce the gas diffusion barrier for the encapsulation due to its special features, such as conformal deposition, defect-free, high-quality layer and excellent uniformity.[3], [4] Since the ALD process is based on sequential self-limiting surface reaction where growth occurs by depositing the material layer by layer, thin films with various properties can be obtained depending on the materials (precursor and reactant) used for deposition as well as the thermodynamic conditions.[5]

In this study, Al₂O₃ was deposited on the a-IGZO TFT by atomic layer deposition (ALD) using trimethylaluminum (TMA) with water or O₃, as the precursor and oxidant, respectively, at low temperature (about 60 °C). First, we fundamentally investigated the characteristics of Al₂O₃ according to the oxidant. The chemical composition of the Al₂O₃ was different depending on the oxidant used in the ALD process (XPS, FT-IR measurement). Based on this, we analyzed the effect of these characteristics on hydrogen barrier properties by using transfer curve (I-V measurement) and stress test of device. As a result, the device in which the Al₂O₃ was deposited exhibited excellent hydrogen barrier properties as compared with the bare device. There was no device degradation after the hydrogen treatment, which suggested the possibility of enhancing the stability by effectively blocking hydrogen in various fields and device reliability in mass production in the future.

Reference

[1] S. I. Oh, G. Choi, H. Hwang, W. Lu, and J. H. Jang, “Hydrogenated IGZO thin-film transistors using high-pressure hydrogen annealing,” IEEE Trans. Electron Devices, vol. 60, no. 8, pp. 2537–2541, 2013.

[2] A. Sato et al., “Amorphous In-Ga-Zn-O thin-film transistor with coplanar homojunction structure,” Thin Solid Films, vol. 518, no. 4, pp. 1309–1313, 2009.

[3] J. Meyer et al., “Al2O3/ZrO2Nanolaminates as ultrahigh gas-diffusion barriersa strategy for reliable encapsulation of organic electronics,” Adv. Mater., vol. 21, no. 18, pp. 1845–1849, 2009.

[4] A. Dameron, S. Davidson, B. Burton, P. Carcia, R. McLean, and S. George, “Gas diffusion barriers on polymers using multilayers fabricated by Al2O3 and rapid SiO2 atomic layer deposition,” J. Phys. Chem. C, vol. 112, pp. 4573–4580, 2008.

[5] S. M. George, A. W. Ott, and J. W. Klaus, “Surface Chemistry for Atomic Layer Growth,” J. Phys. Chem., vol. 100, no. 31, pp. 13121–13131, 1996.